Blackmon, 30, fractured his index finger in Jacksonville's loss to the Miami Dolphins in Week 8. Prior to the injury, he had 28 tackles in eight games. He has one more season on his two-year contract.
The Jaguars signed Fortt to fill the spot on the 53-man roster. The rookie out of California was a fourth-round draft pick of the New Orleans Saints. In 2013, he was a semifinalist for the Butkus Award after recording 64 tackles and 3.5 tackles for loss in nine games.
The Jaguars are 1-8 and play the 6-3 Dallas Cowboys on Sunday.
- Brad Rowland